1. Field of the Invention
The present invention relates to a symbol mapping method, and more particularly, to a method for mapping codeword bits with subcarriers to reduce the number of bits having low reliability when a receiving side uses repetition coding.
2. Discussion of the Related Art
For wireless communication, channel coding is designed to minimize its option considering complexity of a receiving side and a transmitting side. However, in an actual communication system, a code rate lower than that of the designed channel coding may be required considering system coverage and throughput deterioration in a specific position within a cell. In this case, in IEEE 802.16 or 3GPP LTE (Long Term Evolution) system, a repetition coding has been introduced. Particularly, in case of packet retransmission, effect of repetition coding can naturally be obtained through HARQ operation. However, in case of control channel, packets include repetition coding during initial transmission to obtain effect of repetition channel.
FIG. 1 briefly illustrates a procedure of conversion of packets to OFDM transmission signals when channel coding is generally used.
If a source operation is required, a data packet is converted through, for example, an interleaving or ordering scheme (110). The converted bits become codewords by channel coding (120), and before the codewords are converted to actual transmission symbols, channel interleaver (130) is used, if necessary, to obtain diversity. Afterwards, the codeword bits are converted to channel symbols in accordance with a transmission modulation order through channel symbol modulation (140), and can be allocated from a channel symbol mapper (150) to a subcarrier position. If MIMO (Multi-Input Multi-Outout) operation (160) is included in a mapping procedure (170) of mapping channel symbols with OFDM symbols, the MIMO operation (160) may be performed before or after the channel symbols are mapped with subcarriers.
In this case, repetition coding may be performed in the channel coding procedure (120), or may be performed after the bits are converted to the channel symbols (140). When repetition coding is performed in a unit of codeword bits, saving effect of subcarriers can be obtained if the number of codeword bits does not reach a multiple of the number of channel modulation bits.
After the channel coding codewords generated as above are received by a receiving side, they are decoded to the original data packet by a decoding algorithm. However, for optimal decoding, all codeword bits should have same reliability, or reliability should be distributed in accordance with a structure of specific channel coding. If not so, suboptimal throughput is obtained. This is applied regardless of the fact that repetition is performed in a bit level or a symbol level.
If repetition coding is used, repetition coding starts with repetition of original codewords. Repetition coding can be comprised of bit level repetition. Bit level repetition can be performed in a source bit type of FIG. 2 or a codeword bit type of FIG. 3. As illustrated in FIG. 4, after codewords are converted to transmission symbols, a repetition method of the transmission symbols may be used.
In FIG. 2 to FIG. 4, difference in actual throughput is determined by total energy used in transmission. If total code rates are uniformly provided, similar throughput is obtained regardless of the methods in FIG. 2 to FIG. 4. However, if repetition is performed in a bit level, it is advantageous in that subcarriers can be used more effectively when the system transmits a signal.
If repetition coding is used, channel variation of each bit should be minimized so that a receiving side obtains optimal diversity effect. However, in an actual wireless communication channel, channel status is varied per subcarrier and reliability of codeword bits is varied depending on gain. Namely, reliability of specific code bits may be deteriorated by deep fading of channel. Accordingly, the transmitting side should properly set a mapping mode for mapping codeword bits with subcarriers, considering such channel variation.
However, in case of the current IEEE 802.16 system, although repetition is defined in a codeword bit level, there is no specific mapping order after codeword bits are converted to channel symbols, and repetition simply follows a symbol mapping order of the whole system. Particularly, if codewords become a multiple of a specific length, effect according to repetition is simply changed to chase combining, and very poor decoding throughput is caused.